Superman and Batman eventually team up to take on the most dangerous "man" of all: MANKIND.
Oh wait, no. They fight a huge glowing-eyed monster named Doomsday. Which shouldn't have been much of a surprise to anybody. When you name your kid Doomsday, he's not going to grow up to be an accountant.
Though if you name him Doomsdai, he might turn out to be a stripper instead.
Doomsday is nearly invincible, but he does have one weakness -- an Achilles Heel, a chink in the armor, a metaphorical Kryptonite ... actual Kryptonite.
A danger to both Kryptonians and lazy screenwriters.
So Superman knows what HE and ONLY HE ALONE has to do: take one of Batman's Kryptonite spears and fly it right into CPA Doomsday's chest hole, even though it might kill Superman too!
The Obvious Solution:
Wait ... couldn't Superman have done anything else with the spear? The dude can throw objects into space, and it never dawned on him to try tossing the spear at Doomsday?
Like that time he used his powers to pander to r/atheism.